This invention relates in general to systems for use in protecting structures against damage due to earthquakes and, more specifically, structural supports for buildings to substantially isolate the buildings from earthquake forces.
Many earthquakes occur around the world each year causing great losses in lives and property damage. In areas of high earthquake risk, or "seismic zones" near geological faults, it is imperative that buildings be constructed to resist earthquake damage. When a building is supported on or embedded in the ground, an earthquake creates oscillatory movements, primarily in a horizontal plane together with vertical vibrational movements. The oscillatory movements may be greatest in a single direction, as where a land area slips in a horizontal direction relative to adjacent land on the opposite side of a fault. Thus, in addition to oscillatory movements, movement in one direction may be sufficient to displace a building in that direction.
A number of different foundation arrangements and building supports have been designed to reduce and attempt to eliminate earthquake damage. Many of these are effective with minor earth movements. Where earthquake accelerations are beyond the design limits of the supports, great damage may result. Further, in some cases the land sinks or rises, a directional force may be applied which is beyond the capability of known protective supports, which tend to react only substantially equal oscillatory forces.
A number of earthquake resistant supports use friction plates such as are described by Furchak et al in U.S. Pat. No. 4,238,137 and resilient interconnecting members such as are disclosed by Fyfe et al in U.S. Pat. No. 4,617,769 to allow a degree of oscillatory movement. Others utilize spring systems such as described by Suh in U.S. Pat. No. 3,761,068, sometimes in combination with friction plates such as described by Fujimoto et al in U.S. Pat. No. 4,599,834. Fluid filled bags as energy absorbers have been proposed by others, such as Aquilar in U.S. Pat. No. 4,266,379 and some have gone so far as to essentially float a building on a liquid, as described, for example, by Kalpins in U.S. Pat. No. 3,986,367. Hydraulic cylinder type earthquake energy absorbing systems have been proposed by Valencia in U.S. Pat. No. 4,587,773.
While many of these prior systems are somewhat effective against forces produced by moderate earthquakes, damage may still occur. Also, these systems tend to be complex and expensive, limiting their use in poorer countries. Many of these systems only resist oscillatory movement and are ineffective or less effective where major forces are applied in one direction, such as the case of land sinking or slipping near the building or near a fault where land on opposite sides has a relative longitudinal movement.
Thus, there is a continuing need for simple, low-cost, effective supports for buildings in earthquake prone areas to protect against all earthquake generated movements.